RecycledSkbPool: force lock to relax

This spinlock does a lot of spinning.  Instead of doing /* nothing */, make
it relax instead.  I believe this also means we can make _lock non-volatile
but that can be a separate commit.  In the future, it might be worth just
making this a Spinlock.
RecycledSkbPool: make _lock non-volatile

Since click_relax_fence is a memory barrier, _lock does not need to be
volatile.  All other uses are with atomic operations.
Merge pull request #103 from pallas/skbmgr-relax

RecycledSkbPool: force lock to relax